Phase-locked loop (PLL) circuits are well-known in the field of communication systems. They are also they are also commonly used in frequency generating circuits (synthesizers) where quality (accuracy, temperature stability, jitter) of one oscillator is improved by locking to a second higher quality oscillator. The typical task of a PLL is to reproduce and track an original signal while removing as much of the noise as possible. Because of this, they are often used as narrow band filters in low noise satellites communications.
A Phase-Frequency Detector (PFD) is a basic building block of a conventional PLL. Such a conventional PLL is shown in FIG. 1 and, in addition to a PFD, comprises a voltage-controlled oscillator (VCO), a frequency divider, a charge pump (CP) and a loop filter. Here, the CP is an extension of the PFD and is thus analysed (and labelled) in combination with the PFD.
FIG. 1 also shows the various accompanying noise sources in the conventional PLL circuit.
The transfer function of the PLL relates the output phase of the reference signal to the output phase of the VCO. The transfer function of the noise sources present in the different blocks can have a high pass and a low pass characteristic depending on the block being analysed. From the VCOs point, its phase noise has a high pass characteristic to the output of the PLL. From the rest of the blocks it has a low pass characteristic. Therefore, the in-band phase noise floor of the PLL is determined by the noises of the: crystal oscillator's phase noise φX, reference divider's phase noise φref, main divider's phase noise φd, phase-frequency detector's phase noise φpd, charge pump current noise inp and loop filter voltage noise Vnf, and can be expressed as the following equation (Equation 1):φin-band2=φX2+φref2+φd2+φpd2+φLPg2+φCP2 [dBc/Hz].  (1)
The in-band noise floor is important because it sets the noise floor for the receiving signal. Assuming a good low phase noise crystal oscillator and a low noise frequency divider, the predominant in-band noise contributor is the PFD/CP block.
The CP current noise can be decreased on a circuit level. For example, using bipolar instead of MOSFET current mirrors can help lower the 1/f noise. Resistive emitter degeneration in a current mirror can also help reduce the transistors current noise. Another approach to reducing the CP current noise can be taken on a system level. Here, to analyse this, the output noise of the charge pump inp(f) is referred back to the input of the PFD/CP (because it has a low pass transfer function) as phase noise as written in equation 2 below:
                                          PN            CP                    =                                    10              ⁢                              log                ⁡                                  (                                      φ                    CP                    2                                    )                                                      =                                          10                ⁢                                                      log                    ⁡                                          (                                                                                                    i                            np                                                    ⁡                                                      (                            f                            )                                                                                                    K                          pd                                                                    )                                                        2                                            =                              20                ⁢                                                      log                    ⁡                                          (                                              2                        ⁢                        π                        ⁢                                                                                                                                                                    ⁢                                                                                          i                                np                                                            ⁢                                                              (                                f                                )                                                                                                                                          I                            CP                                                                                              )                                                        ⁡                                      [                                          dBc                      ⁢                                              /                                            ⁢                      Hz                                        ]                                                                                      ,                            (        2        )            
where Kpd=ICP/2π is the gain of the PFD/CP block and ICP is the dc value of the CP current. From this, it can be seen that a higher value of Kpd will result in lower system noise. Accordingly, the typical approach to increase Kpd is to increase ICP, nut this has the drawback of increasing power consumption and decreasing the voltage headroom of the CP output, as well as increasing the noise of the charge pump inp(f).